Specifically your BAB changes while you are flurrying, and since BAB directly impacts how many swings you get, this needs to be modified directly instead of just applying a generic +/- to attack. (I don't make the rules, so don't fuss at me
Yeah, but the text says the BAB change doesn't effect anything else, like number of attacks, it just provides the to hit bonus for the flurry.
Flurry of Blows (Ex)
Starting at 1st level, a monk can make a flurry of blows as a full-attack action. When doing so he may make one additional attack using any combination of unarmed strikes or attacks with a special monk weapon (kama, nunchaku, quarterstaff, sai, shuriken, and siangham) as if using the Two-Weapon Fighting feat (even if the monk does not meet the prerequisites for the feat). For the purpose of these attacks, the monk's base attack bonus is equal to his monk level. For all other purposes, such as qualifying for a feat or a prestige class, the monk uses his normal base attack bonus.
At 8th level, the monk can make two additional attacks when he uses flurry of blows, as if using Improved Two-Weapon Fighting (even if the monk does not meet the prerequisites for the feat).
At 15th level, the monk can make three additional attacks using flurry of blows, as if using Greater Two-Weapon Fighting (even if the monk does not meet the prerequisites for the feat).
A monk applies his full Strength bonus to his damage rolls for all successful attacks made with flurry of blows, whether the attacks are made with an off-hand or with a weapon wielded in both hands. A monk may substitute disarm, sunder, and trip combat maneuvers for unarmed attacks as part of a flurry of blows. A monk cannot use any weapon other than an unarmed strike or a special monk weapon as part of a flurry of blows. A monk with natural weapons cannot use such weapons as part of a flurry of blows, nor can he make natural attacks in addition to his flurry of blows attacks.
To me, it doesn't make sense to give an extra attack and then increase the number of attacks based on BAB. So, it just boils down to an attack bonus based on the monk level.